Methods for measuring the alcohol content of a subject's expired breath to determine the alcohol concentration in the subject's blood are based on the proposition that alveolar air from the lungs has an alcohol content that is in equilibrium with the alcohol content of the blood. Only deep lung air will give an accurate measure, and, therefore, some means must be established to make sure that air in the mouth and upper respiratory tract has been discarded before making the measurement.
With infrared (IR) alcohol detectors now in commercial use, the subject has been required to blow at a minimum velocity and to sustain that velocity for a predetermined length of time before the IR measurement is made. This arrangement ensures that all subjects must deliver at least a minimum volume before the test will be valid. A problem with this method, is that a person who blows very hard may expire the predetermined minimum volume, generally considered to be about 800 to 1000 cc., before the time interval has elapsed. The problem is aggravated if the instrument's breath inlet system has a small resistance to the subject's expelling his breath. Subjects with small lung capacity can therefore deliver a desired minimum volume by blowing hard in less than the required time interval, so that the measurement is not triggered, and yet still be delivering deep lung air at the end of their exhalation. Thus, the same volume of air may be delivered over two different time intervals, but in many conventional devices, because of the use of a timing mechanism, a sample from the breath of a person with small lung capacity, who blows hard, may in effect be rejected. Although the measure of breath sample is sometimes expressed in terms of velocity, this would be interchangeable with pressure if the breath inlet system has a fixed restriction, because a particular velocity would require a given pressure at the inlet.
As shown in FIG. 1, the output of an IR instrument in response to breath samples delivered at a high pressure (A) and at low pressure (B) will reach the same final value. Accordingly, a determination of breath volume, without the requirement that it be delivered over a certain time interval is a desirable feature to establish the minimum requirement for completing breath alcohol measurement.
It is also highly desirable for evidentiary purposes to establish a base line, an alcohol standard, and a blank test record at each breath test.
One of the objects of this invention is to provide apparatus and method for measuring volume of a breath sample to trigger measurement of the alcohol content by an alcohol detector, without regard to time interval.
Another object is to provide an instrument the measurement of alcohol of which is of highly reliable evidentiary value.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.